1. Field of the Invention
The present invention relates to an illumination optical apparatus for uniformly illuminating an object using a collimated light source such as laser and, more particularly, to an illumination optical apparatus suitable for an exposure apparatus used for manufacturing a semiconductor element such as an IC.
2. Related Background Art
Example of a conventional illumination apparatus suitable for use in an exposure apparatus for manufacturing a semiconductor element such as an IC include: an apparatus adopting an elliptic mirror and a single optical integrator as described in Japanese Laid-open Patent Application No. 81813/1981, and an apparatus using two serially arranged optical integrators for improving uniformity of illumination as described in U.S. Pat. No. 4,497,015. However, in these illumination optical apparatus, an ultra high-pressure mercury lamp is used as a light source. For this reason, the light emission characteristic in the short wavelength range is insufficient, and only a small amount of light can be obtained. Thus, it is proposed that a laser be used as a high-output light source for a short wavelength range instead of the ultra high-pressure mercury lamp. An apparatus incorporating a laser light source is described in U.S. Pat. No. 4,619,508.
However, when an excimer laser or a YAG laser is used as a light source, the laser power is very strong, and a focal point becomes a very small spot. For this reason, a spot with a very high energy density is formed, and lens may occur due to reflection inside the. Also, ghost light may appear on an object to be illuminated. In particular, if a focal point is formed at an edge of an aperture stop arranged in an illumination optical system, strong scattered light is generated therefrom as flare. Therefore, it is difficult to maintain a uniform illuminance at the object. In addition, lens damage may be caused by the flare.